Technical Field
The present disclosure refers to a vehicle positioning apparatus.
The present disclosure refers also to a global navigation satellite system of the hybrid-inertial type, comprising such positioning apparatus.
Further, the present disclosure refers to a method of detecting satellite signals of a global navigation satellite system.
Description of the Related Art
As it is known, the global navigation satellite systems or GNSSs, English acronym standing for Global Navigation Satellite Systems, are widely used for locating in real time the position of a vehicle and guiding a vehicle to a requested destination.
The electromagnetic signals supplied by several satellites, are received by a GNSS receiver, are formatted and processed and are capable of supplying an accurate estimate of the vehicle position in terms of: latitude, longitude and altitude above sea level.
In many real cases, for example in an urban environment, possible noises, radio frequency (RF) interferences, reflections of signals, errors in the transmission or spurious transmissions of signals prevent or degrade the integrity of the signals received from the satellites and/or the continuity of the received data, and therefore the positions defined by the GNSS receiver are not correct. The latitude, longitude and altitude values uniquely calculated from the satellite signals can sometimes have an error of tens of meters. In the standard navigation applications, the calculated latitude and longitude are matched on a one-level (two-dimensional) map and, in the presence of the above mentioned errors, this is difficult. In case of multi-level roads, the matching problem is exacerbated by the introduction of the vertical dimension, and therefore it is fundamental to have the height value of the vehicle, for matching the position on the correct level of the map and for obtaining a correct navigation. Moreover, in indoor environments such as parking lots, underground garages, tunnels and similar, the electromagnetic signals supplied by the satellites can completely disappear or are only partially received, and the GNSS receiver is not capable of providing in real time information of correct positions.
A solution provides the use of a Dead Reckoning positioning system, known as DR, supplying the position of an user in a local reference system (North-East), by considering as a reference a known position calculated in the past and updating it until the current instant by measuring the displacement of the user for example by identifying the linear traveled distance and possible changes of direction. The DR systems use an inertial measurement unit, IMU, comprising inertial sensors which generally are mechanical MEMS (acronym standing for Micro Electro Mechanical System) sensors which are economical.
Among the used MEMS sensors, there are accelerometers and monoaxial and triaxial gyroscopes. The accelerometer, when suitably installed, enables determination of the absolute orientation of the inertial measuring unit and therefore of the vehicle with reference to the earth gravitational field. The gyroscope enables detection of possible relative orientation variations (rotations) of the inertial measuring unit, IMU, and therefore of the vehicle.
The hybrid inertial navigation systems enable integration of the satellite signals received by the GNSS receiver and the signals received by the movement sensors and use estimate systems for compensating the drawbacks shown by both such systems.
Despite the fact that the inertial sensors are adequate under several aspects, they are not very reliable in the long term. Even though the inertial sensors are reliable in the short term and insensitive to environmental problems typically troubling the electromagnetic signals, they are prone to errors (caused by the measuring noise and calibration errors) building up with time. Therefore, their performances deteriorate in the long term and the supplied information, such as the position, in other words the longitude and latitude, the altitude, and the speed or heading of the vehicle, are not reliable.
Further, it is well to observe that a navigation device with signals received from a satellites constellation is reliable in the long term, however the obtained data can be incorrect in the short term, for example due to an incorrect or absent reception of signals. The type of contributions given by the two systems is complementary: the GNSS positioning can be incorrect in the short term but is generally accurate in the long term, while the MEMS sensors enable very precise calculation of the displacement in the short term, getting less reliable if they are considered in the long term.
The subject matter discussed in the Background section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in the Background section. Along these lines, any recognition of problems in the prior art discussed in Background section or associated with such subject matter should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion of any subject matter in the Background section should be treated as part of the inventor's approach to the particular problem, which in and of itself may also be inventive.